Windows 10 will apparently be free to users of Windows 8 and 8.1 although this could change before Windows 10 is actually released. If you have just been using the Tech Previews then this doesn't automatically qualify you for a free copy of Win 10. You must be a Windows 8 or 8.1 user too...

Windows 10 TH is aimed only to get the opinion of tech guys about its look and performance. After then, general users will experience the taste of Windows 10 and it will be named consumer preview. Looking responses on both levels Microsoft will release the original version and it will not be free you know. After launch of Windows 10 both the links will be dead.